1. Field of the Invention
This invention relates to the fabrication of integrated circuits, such as of the thin film integrated circuit type and, more particularly, to the fabrication of circuit crossovers, and the subsequent bonding thereof to respectively associated bond sites of the circuit.
2. Description of the Prior Art
With the ever increasing complexity and density of integrated circuitry processed and/or fabricated on ceramic substrates, the need for interconnecting circuit "crossovers", of three-dimensional configuration, have often proven essential in order to optimize the utilization of substrate surface area. As the name implies, a crossover essentially comprises a bridge-like element which allows different portions of a circuit, such as two spaced bonding pads or sites, to be interconnected, while an intermediate portion of the crossover extends out of the otherwise uniform plane of the circuit so as to bridge an intervening metallized circuit path or element.
The complexity of the problem of fabricating and bonding an array of crossovers to an otherwise completely fabricated integrated circuit may be appreciated by the following typical example. In one representative hybrid integrated circuit pack employed in an electronic telephone switching system, the supporting ceramic substrate measures 31/4".times.4", with not only dozens of individual silicon integrated circuits and 92 lead connections, but over 4,000 crossovers. Each crossover typically measures from 4 to 6 mils in width, 40 to 80 mils in length, and 25 to 50 microns in thickness, with an intermediate bowed region having an arch height on the order of 3 mils, and with the spacings between crossovers often being as close as 12 mils, center to center.
One prior method of fabricating such crossovers is dislcosed in U.S. Pat. No. 3,461,524, of M. P. Lepselter, assigned to Bell Telephone Laboratories. In that method, generally referred to as a plated beam method, an intermediate conductive layer, such as of copper, is initially deposited over each circuit element which is to be crossed over, or bridged, and an outer conductive layer, such as of gold, is then deposited over the intermediate layer and onto selected areas of different portions of the circuit which are to be interconnected on opposite sides of each circuit element to be bridged. The intermidiate material is then removed so as to leave an air dielectric between each crossover and the bridged circuit element. If desired, a permanent solid dielectric can be deposited between the formed crossovers and respectively bridged circuit elements. A similar technique for fabricating plated beam type crossovers is disclosed in an article entitled "Batch Bonded Crossovers for Thin Film Circuits", published in the Western Electric Engineer, Vol. XX, No. 2, April 1976.
U.S. Pat. No. 4,054,484 of N. G. Lesh et al., also assigned to Bell Telephone Laboratories, discloses a crossover fabrication technique wherein evaporated layers of titanium and copper are used as base layers to build up a beam-type crossover spacing layer. A nickel protective layer is formed over the evaporated layers, as well as over the circuit areas initially formed on the substrate. A copper spacing layer is then plated over the nickel layer. Spaced pairs of pillar holes are then etched in selected areas of both the copper spacing layer and the nickel protective layer to expose spaced regions of the initially formed circuit therebelow to be interconnected. This is followed by forming the desired interconnecting gold crossovers on the copper spacing layer. The copper spacing layer is finally removed by an etchant which preferentially attacks the copper. The nickel protective layer and the copper base layers are also removed, preferably with the same etchant.
While the aforementioned plated beam crossover techniques are capable of producing well-defined and extremely minute crossovers, such techniques do give rise to several potentially serious manufacturing problems. First, the processing steps required are both time consuming and costly. Secondly, the etchants required to remove selected material to form the plated beam type crossovers may often be incompatible with other circuit materials and/or devices forming the composite circuit, thereby possibly resulting in damage to the circuit, even when great care is taken in the fabricating, as well as handling, of the circuit. With respect to handling, it is readily apparent that the more times the circuit and/or crossovers must be manipulated in carrying out the necessary processing steps, the more susceptible is the circuit to damage. Thirdly, it will be appreciated that there is no simple, reliable way of testing whether plated beam type crossovers are adequately secured to the circuit, as it would be extremely difficult, as well as time consuming and expensive, to test the bond strength of such crossovers on either an individual or random basis. Moreover, even if random testing were employed, it is very difficult to perform peel tests on plated-beam type crossovers without either destroying the bonds, or otherwise damaging the crossovers.
As a result of the problems and costs encountered in fabricating plated-beam type crossovers, a batch bonded method was developed and is disclosed in J. A. Burns et al. U.S. Pat. No. 3,762,040, assigned to the same assignee as the present invention. In accordance with one preferred version of this last-mentioned method, an array of metallic crossovers are initially formed as minute bars, i.e., without arched regions, on a carrier member, such as a copper-polyimide laminated film, in accordance with any one of a number of conventional mask-defined, metal plating techniques. The resultant array of two-dimensional plated crossovers are thereafter formed, in conjunction with a specially constructed backing member (having an array of slos that correspond with the generated array of crossovers), by an extrusion or deformation technique either prior to or during the subsequent bonding of the crossovers to a circuit. This technique is also disclosed in the aforementioned Western Electric Engineer article.
J. A. Burns U.S. Pat. No. 3,729,816, also assigned to the same assignee as the present invention, discloses the use of a batch bonded technique for generating temporary crossovers for use in the electrical testing of portions of a thin film circuit during the fabrication thereof.
The prior batch bonded crossover method has a number of advantages over the plated-beam crossover method. First, the number of photoresist, plating and etching steps are considerably reduced. This leads to lower fabrication costs, and often higher yields. Higher yields are also more often realized with the prior batch bonded versus plated beam crossover fabrication method because the former method is carried out completely independently of the considerably more expensive thin film circuit-generating method. As a result, both the fabricated crossover array and the associated integrated circuit may be independently examined for defects prior to the crossovers being bonded to the circuit as the last, or one of the last, steps involved in the fabrication of a complete, composite circuit.
Notwithstanding the significant advantages realized by utilizing the prior batch bonded crossover method, the fact remains that it has nevertheless still proven to be a relatively costly method, primarily because of not only the necessity of having to use a rather expensive copper-polyimide laminate carrier, but because of the attendant photoresist and chemical etching steps required to generate the array of crossovers, not to mention the additional steps involved to form the arched regions thereof. In addition, when the etched window backing member, employed to form the crossover arches, is also used as a temporary frame for the copper-polyimide film, and is preferably made of molybdenum (for reasons discussed in greater detail hereinafter), the cumulative exposure thereof to the necessary copper etchant unfortunately has been found to limit its life to about fifty operating cycles. As such backing members, particularly when formed with complex arrays of precision formed etched windows, are rather expensive, it is seen that their periodic replacement costs alone may constitute an appreciable portion of the overall costs of fabricating thin film integrated circuits, with batch-bonded crossovers of the type in question, in high volume manufacturing operations.